Monitoring toxic gases is a great concern in relation to environmental pollution, occupational health, and industrial emission control. Known methods and apparatuses have been developed to detect the presence of gases. For example, gas chromatography, ion chromatography, electrolytic conductivity detection, and conductometric measurement are typically used to detect gases. However, these manners for detecting gases have generally been expensive and cumbersome.
Electrochemical sensors were provided to overcome these limitations. Electrochemical sensors typically operate at room temperature, provide a signal which varies linearly with concentrations of analyte species, have short response time, and exhibit acceptable sensitivity with high durability. In addition, electrochemical sensors are compact and can be used for continuous monitoring.
Known electrochemical gas sensors typically include gas diffusion electrodes in contact with a conductive medium, such as electrolyte. Sensors typically measure gas by diffusing it through an electrolyte. The electrolyte is in contact with electrodes, which in turn permits a measurement of current flow between them. The gas generally undergoes either oxidation or reduction to produce yielding a signal in the form of a current, which is typically larger than the background current. Hence, there is generally a correlation between a measurement of current flow and the concentration of a particular gas.
The cornerstone of these sensors generally has been on optimizing the electrode/gas/electrolyte interface in order to achieve higher sensitivity. However, known sensors having thick films of electrolyte require longer gas diffusion times and, therefore, have slower response times.
Recently, planar thin film sensors have been developed by constructing three planar electrodes on alumina substrate and covering them with a thin polymer electrolyte, such as Nafion. J. A. Cox and K. S. Alber, Amperometric Gas Phase Sensor for the Determination of Ammonia in a Solid State Cell Prepared by a Sol-Gel Process, 143, No. 7 J. Electrochem. Soc. L126-L128 (1996) developed a solid state cell in which microelectrode arrays were coated with a film of vanadium oxide xerogel for detection of ammonia. However, this film needs to be soaked in a solution in order to provide ionic conductivity. Soaking the electrolyte tends to cause a thick layer of electrolytic solution, which floods the electrode surface. Flooding negatively affects sensor response time due to the thick layer of electrolytic solution through which gas must diffuse.
Another contributor to flooding is an electrode surface having crevices or pores, where electrolytic solution may accumulate. Because a difference of only a fraction of a micrometer differentiates between a desirably thin film of electrolyte and a thick film of electrolytic solution that floods the electrodes, crevices or pores in the surface may affect sensor sensitivity. Similarly, wicking, where solution seeps into or beneath the electrode surface, negatively affects sensor response time for the same reasons as crevices/pores.
Lower temperatures may also negatively affect an electrochemical sensor's efficiency for the response time is typically slower. This may be due to a host of factors, including a higher viscosity of the solution used to wet the electrolyte.
U.S. Pat. No. 5,716,506 to Maclay et al. (“Maclay”) relates to an electrochemical gas sensor for detecting gases and taking into account relative humidity and air temperature. The patent discloses thin film platinum electrodes for increasing a sensor's sensitivity. The patent also discloses that the electrodes may be electrodeposited, or electroplated in acid. The reference further discloses that electrodes may be sputtered onto a substrate. However, nowhere does Maclay disclose an electrode comprising a thin film for minimizing flooding of the working electrode. Also, Maclay does not disclose an electrode surface that minimizes crevices or porosity for preventing flooding and/or wicking. Further, nowhere does Maclay disclose an acidic solution for use in a sensor operating below 0° C.
What is desired, therefore, is an electrochemical sensor having improved sensitivity and response time. What is also desired is a sensor that prevents flooding and/or wicking of the electrode surface. What is further desired is a sensor operational at lower temperatures.